1. Field of the Invention
The present invention relates to acceleration sensors and manufacturing methods thereof.
2. Description of the Related Art
A known acceleration sensor utilizing piezoelectric ceramics is described in, for example, Japanese Unexamined Patent Application Publication No. 6-273439. The known acceleration sensor includes a piezoelectric element having a bimorph structure. The bimorph piezoelectric element is formed by bonding a pair of piezoelectric ceramic layers so as to face each other, providing an intermediate electrode between the piezoelectric ceramic layers, and providing signal extraction electrodes on the front and back faces of the bimorph piezoelectric element. The bimorph piezoelectric element is housed in and supported by a double beam structure. In the acceleration sensor, a center portion and both end portions of each of the piezoelectric ceramic layers in the longitudinal direction are polarized in opposite directions. Thus, charge generated in the center portion and both end portions can be externally extracted. As a result, the charge extraction efficiency can be improved.
In the bimorph acceleration sensor, it is necessary to differently polarize the center portion and the end portions of each of the piezoelectric ceramic layer. To this end, surface electrodes which are separated from each other are formed on a surface of the piezoelectric ceramic layer, and, after polarization, a connection electrode which completely covers the surface electrodes is formed, whereby a signal extraction electrode is formed. An acceleration sensor which can reduce the burden of two-step electrode formation is proposed in Japanese Unexamined Patent Application Publication No. 8-166401.
In both of the above-described acceleration sensors, the piezoelectric element is formed by the two piezoelectric ceramic layers, and the capacitance of the piezoelectric element is relatively small. Thus, the charge sensitivity is not very high.
In order to improve the charge sensitivity, an acceleration sensor including a piezoelectric element formed by stacking three piezoelectric ceramic layers is proposed (see Japanese Unexamined Patent Application Publication No. 10-62445). In this case, the capacitance is increased by increasing the number of piezoelectric layers being stacked. As a result, the charge sensitivity can be improved. The structure is, however, limited in that the polarization direction cannot be reversed in the same piezoelectric layer. Thus, charge can be extracted only from the center portion of a piezoelectric crystal, and the charge extraction efficiency is not very high.